(1) Field of the Invention
The present invention relates to a structure for a stroke (shock energy) absorbing type intermediate shaft for a vehicular steering column and a method for assembling the same.
(2) Background of the Art
A steering column installed in a vehicle is generally linked to a steering gear box via an intermediate shaft. The intermediate shaft is linked with a universal joint and located at a lower end portion of a steering shaft axially attached to an upper end of a steering wheel. Then, when the vehicle collides with an obstacle at its front portion, the steering gear box is retracted toward the rear of the vehicle to press up the intermediate shaft. If the linked steering shaft is pressed upward, a danger that the vehicle driver will be tightly pressed against the steering wheel is present. The intermediate shaft, therefore, needs to be contractible and to be of a, so called, stroke absorbing type so that a displacement of a steering gear box does not transmit force in an axial direction of the steering shaft.
To meet such demand, the intermediate shaft includes a tubular portion having an approximately elliptic cross section, i.e., parallel straight sides and semi-circular sides in cross section, the tubular portion being linked with a yoke portion constituting the universal joint, and a shaft having a portion fitted into the tubular portion. The portion of the shaft is tightly fitted into the tubular portion via an anti-vibration member made of, e.g., rubber in order to absorb or buffer transmission of vibrations or sounds generated during vehicle operational from the steering gear box.
In the intermediate shaft, the portion of the shaft is contractibly fitted into the tubular portion in its axial direction, and a rotational force rotating the steering shaft with its axis as a center can be transmitted from the tubular portion to the shaft.
Such a structure of the intermediate shaft as described above is exemplified by Japanese Utility Model Registration Application First Publication Showa 61-12876 published on Jan. 25, 1986 and Japanese Utility Model Registration Application First Publication Showa 61-12877 published on the same date.
Another structure of the intermediate shaft has been proposed in a Japanese Patent Application First Publication Showa 53-91236 published on Aug. 10, 1978.
In the other structure of the previously proposed intermediate shaft disclosed in the above-identified Japanese Patent Application First publication, the intermediate shaft includes a shaft tube coaxially fitted into an outer tube portion linked with a lower end of the steering shaft via an elastic member. A shaft body of the intermediate shaft is coaxially fitted into the shaft tube via a groove inscribed on an inner peripheral surface of the shaft tube and via an adhesive. Axial recesses are cut out so as to face each other at one end of the outer tube portion in a radial direction of the outer tube portion. Stoppers engaging the recesses are fitted to the shaft tube, and the other end portion of the outer tube is formed with a joint.
Another structure of an intermediate shaft has been proposed in a Japanese Utility Model Registration Application Second Publication Showa 59-29147 published on Aug. 22, 1984. The structure of the intermediate shaft disclosed in the above-identified Japanese Utility Model Registration Application Second Publication is substantially similar to those disclosed in the above-identified Japanese Utility Model Registration Application First Publications.
Furthermore, still another structure of the previously proposed intermediate shaft includes a yoke portion linked to another yoke portion fixed to the lower end of the steering shaft. A hollow tube having a circular cross section is linked with the yoke portion, and a stretch forming portion is formed in the vicinity of a linked portion between the hollow tube and yoke portion. The intermediate shaft further includes a shaft body having one end formed with an axle portion and the other end formed with a serration to link with the steering gear box. The shaft body is inserted into the hollow tube portion under pressure. The axle portion is provided with a tip having an analogous cross section to the stretch forming portion, and a buffer member such as a rubber member is fixed onto an outer peripheral surface of the axle portion. An inner tube is attached onto an outer peripheral surface of the buffer member. The buffer member is vulcanized and adhered (by baking) to the axle portion and inner tube. Hence, the intermediate shaft described above provides torsion for the hollow tube when the hollow tube is rotated and the buffer member is accordingly rotated and transmits the rotational force to the axle portion while providing shearing force for the buffer member.
When a strong rotational force is applied to the hollow tube, the force is transmitted to the shaft body and the tip of the hollow tube directly connected with the stretch forming portion. Since the buffer member intervenes between the axle portion and hollow tube, the vibrations and sounds transmitted from the shaft body are absorbed or relieved by means of the buffer member so as to interrupt or relieve transmission of such vibrations to the steering shaft.
However, the previously proposed intermediate shaft structures described above have drawbacks.
That is to say, in the stroke absorbing type intermediate shaft disclosed in the Japanese Utility Model Registration Application First Publications Showa 61-12876 and Showa 61-12877 and the Japanese Patent Utility Model Registration Application Second Publication showa 59-29147, the anti-vibration member is quite thin and, therefore, the torsional spring constant is low. Consequently, a so-called, dead zone (insensitive zone) occurs in the intermediate shaft while the vehicle runs straight at high speeds and steering response characteristic to minor steering adjustments at high speed becomes worsened.
Furthermore, since the anti-vibration (buffer) member is directly inserted into the tube under pressure, stress in the inserted portion against the tube becomes lower so that play occurs in the axial and rotational directions of the intermediate shaft.
In the previously proposed structure of the intermediate shaft disclosed in the Japanese Patent Application First Publication Showa 53-91236, the following problems occur.
(1) Since a structure of fitting stoppers into the shaft tube utilizes a circular form in cross section, the stoppers will rotate themselves with no cooperational rotation if the stoppers are twisted with a large force. This means that under some conditions transmission of steering torque between the tube and shaft tube may not be carried out.
(2) Owing to deterioration of the elastic member and peeling of the adhered portion between the shaft tube and shaft, the stoppers will often draw out of the tube when not tightly fixed thereto.
(3) Since the recesses engaging the stoppers are opened in a single direction, it is necessary to enlarge the thickness of the recesses in order to secure torsional strength of the stoppers. Consequently, workability of the intermediate shaft becomes worsened and the whole weight thereof becomes heavy.
(4) Since the cross sectional form of the elastic member is circular, error easily occurs when inserting the shaft, into the tube shaft and the positioning accuracy of the stoppers thus may easily be compromised.